Body image issues in the entertainment world being what they are, I think Mariko is probably under loads of pressure to watch her weight.
The weight charts in Japan (of what is "normal" or ideal weight) would be considered very severe in other countries.
This is Ryo Shihono, a gravure idol, who is very thin

Mariko is not as thin as Ryo

But no doubt,she has lost weight since the early days

So along with the pressures of performance, there is the appearance pressure. It would be nice if idol groups had nutrition coaches (who knows, maybe they do?) and counsellors to make sure that they do not fall down under too much fan pressure or peer pressure